Publications


SELF-GRAVITY, RESONANCES AND ORBITAL DIFFUSION IN STELLAR DISKS

ASTROPHYSICAL JOURNAL 806 (2015) ARTN 117

J-B Fouvry, J Binney, C Pichon


A fast algorithm for estimating actions in triaxial potentials

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 447 (2015) 2479-2496

JL Sanders, J Binney


Early flattening of dark matter cusps in dwarf spheroidal galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 446 (2015) 1820-1828

C Nipoti, J Binney


On the interpretation of dark matter self-interactions in Abell 3827

ArXiv (2015)

F Kahlhoefer, K Schmidt-Hoberg, J Kummer, S Sarkar

Self-interactions of dark matter particles can potentially lead to an observable separation between the dark matter halo and the stars of a galaxy moving through a region of large dark matter density. Such a separation has recently been observed in a galaxy falling into the core of the galaxy cluster Abell 3827. We estimated the DM self-interaction cross section needed to reproduce the observed effects and find that the sensitivity of Abell 3827 has been significantly overestimated in a previous study. Our corrected estimate is $\tilde{\sigma}/m_\text{DM} \sim 3\:\text{cm}^2\:\text{g}^{-1}$ when self-interactions result in an effective drag force and $\sigma/m_\text{DM} \sim 1.5\:\text{cm}^2\:\text{g}^{-1}$ for the case of contact interactions, in some tension with previous upper bounds.


Evidence of locally enhanced target heating due to instabilities of counter-streaming fast electron beams

PHYSICS OF PLASMAS 22 (2015) ARTN 020701

P Koester, N Booth, CA Cecchetti, H Chen, RG Evans, G Gregori, L Labate, T Levato, B Li, M Makita, J Mithen, CD Murphy, M Notley, R Pattathil, D Riley, N Woolsey, LA Gizzi


Simplified Models for Dark Matter Searches at the LHC

ArXiv (2015)

J Abdallah, H Araujo, A Arbey, A Ashkenazi, A Belyaev, J Berger, C Boehm, A Boveia, A Brennan, J Brooke, O Buchmueller, M Buckley, G Busoni, L Calibbi, S Chauhan, N Daci, G Davies, ID Bruyn, PD Jong, AD Roeck, KD Vries, DD Re, AD Simone, AD Simone, C Doglioni, M Dolan, H Dreiner, J Ellis, S Eno, E Etzion, M Fairbairn, B Feldstein, H Flaecher, F Eric, M-H Genest, L Gouskos, J Gramling, U Haisch, R Harnik, A Hibbs, S Hoh, W Hopkins, V Ippolito, T Jacques, F Kahlhoefer, VV Khoze, R Kirk, A Korn, K Kotov, S Kunori, G Landsberg, S Liem, T Lin, S Lowette, R Lucas, L Malgeri, S Malik, C McCabe, AS Mete, E Morgante, S Mrenna, Y Nakahama, D Newbold, K Nordstrom, P Pani, M Papucci, S Pataraia, B Penning, D Pinna, G Polesello, D Racco, E Re, AW Riotto, T Rizzo, D Salek, S Sarkar, S Schramm, P Skubic, O Slone, J Smirnov, Y Soreq, T Sumner, TMP Tait, M Thomas, I Tomalin, C Tunnell, A Vichi, T Volansky, N Weiner, SM West, M Wielers, S Worm, I Yavin, B Zaldivar, N Zhou, K Zurek

This document outlines a set of simplified models for dark matter and its interactions with Standard Model particles. It is intended to summarize the main characteristics that these simplified models have when applied to dark matter searches at the LHC, and to provide a number of useful expressions for reference. The list of models includes both s-channel and t-channel scenarios. For s-channel, spin-0 and spin-1 mediation is discussed, and also realizations where the Higgs particle provides a portal between the dark and visible sectors. The guiding principles underpinning the proposed simplified models are spelled out, and some suggestions for implementation are presented.


Fluctuation-dissipation relations for a plasma-kinetic Langevin equation

Journal of Plasma Physics 81 (2015)

A Kanekar, AA Schekochihin, W Dorland, NF Loureiro

© 2014 Cambridge University Press. A linearised kinetic equation describing electrostatic perturbations of a Maxwellian equilibrium in a weakly collisional plasma forced by a random source is considered. The problem is treated as a kinetic analogue of the Langevin equation and the corresponding fluctuation-dissipation relations are derived. The kinetic fluctuation-dissipation relation reduces to the standard fluid one in the regime where the Landau damping rate is small and the system has no real frequency; in this case the simplest possible Landau-fluid closure of the kinetic equation coincides with the standard Langevin equation. Phase mixing of density fluctuations and emergence of fine scales in velocity space is diagnosed as a constant flux of free energy in Hermite space; the fluctuation-dissipation relations for the perturbations of the distribution function are derived, in the form of a universal expression for the Hermite spectrum of the free energy. Finite-collisionality effects are included. This work is aimed at establishing the simplest fluctuation-dissipation relations for a kinetic plasma, clarifying the connection between Landau and Hermite-space formalisms, and setting a benchmark case for a study of phase mixing in turbulent plasmas.


Black hole evolution: I. Supernova-regulated black hole growth

ArXiv (2015)

Y Dubois, M Volonteri, J Silk, J Devriendt, A Slyz, R Teyssier

The growth of a supermassive black hole (BH) is determined by how much gas the host galaxy is able to feed it, which in turn is controlled by the cosmic environment, through galaxy mergers and accretion of cosmic flows that time how galaxies obtain their gas, but also by internal processes in the galaxy, such as star formation and feedback from stars and the BH itself. In this paper, we study the growth of a 10^12 Msun halo at z=2, which is the progenitor of al group of galaxies at z=0, and of its central BH by means of a high-resolution zoomed cosmological simulation, the Seth simulation. We study the evolution of the BH driven by the accretion of cold gas in the galaxy, and explore the efficiency of the feedback from supernovae (SNe). For a relatively inefficient energy input from SNe, the BH grows at the Eddington rate from early times, and reaches self-regulation once it is massive enough. We find that at early cosmic times z>3.5, efficient feedback from SNe forbids the formation of a settled disc as well as the accumulation of dense cold gas in the vicinity of the BH and starves the central compact object. As the galaxy and its halo accumulate mass, they become able to confine the nuclear inflows provided by major mergers and the BH grows at a sustained near-to-Eddington accretion rate. We argue that this mechanism should be ubiquitous amongst low-mass galaxies, corresponding to galaxies with a stellar mass below <10^9 Msun in our simulations.


Electron-ion temperature equilibration in warm dense tantalum

HIGH ENERGY DENSITY PHYSICS 14 (2015) 1-5

NJ Hartley, P Belancourt, DA Chapman, T Doeppner, RP Drake, DO Gericke, SH Glenzer, D Khaghani, S LePape, T Ma, P Neumayer, A Pak, L Peters, S Richardson, J Vorberger, TG White, G Gregori


Black hole evolution: II. Spinning black holes in a supernova-driven turbulent interstellar medium

ArXiv (2014)

Y Dubois, M Volonteri, J Silk, J Devriendt, A Slyz

Supermassive black holes (BH) accrete gas from their surroundings and coalesce with companions during galaxy mergers, and both processes change the BH mass and spin. By means of high-resolution hydrodynamical simulations of galaxies, either idealised or embedded within the cosmic web, we explore the effects of interstellar gas dynamics and external perturbations on BH spin evolution. All these physical quantities were evolved on-the-fly in a self-consistent manner. We use a `maximal' model to describe the turbulence induced by stellar feedback to highlight its impact on the angular momentum of the gas accreted by the BH. Periods of intense star formation are followed by phases where stellar feedback drives large-scale outflows and hot bubbles. We find that BH accretion is synchronised with star formation, as only when gas is cold and dense do both processes take place. During such periods, gas motion is dominated by consistent rotation. On the other hand, when stellar feedback becomes substantial, turbulent motion randomises gas angular momentum. However BH accretion is strongly suppressed in that case, as cold and dense gas is lacking. In our cosmological simulation, at very early times (z>6), the galactic disc has not yet settled and no preferred direction exists for the angular momentum of the accreted gas, so the BH spin remains low. As the gas settles into a disc (6>z>3), the BH spin then rapidly reaches its maximal value. At lower redshifts (z<3), even when galaxy mergers flip the direction of the angular momentum of the accreted gas, causing it to counter-rotate, the BH spin magnitude only decreases modestly and temporarily. Should this be a typical evolution scenario for BH, it potentially has dramatic consequences regarding their origin and assembly, as accretion on maximally spinning BH embedded in thin Shakura-Sunyaev disc is significantly reduced.


Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

NATURE PHYSICS 10 (2014) 520-524

J Meinecke, HW Doyle, F Miniati, AR Bell, R Bingham, R Crowston, RP Drake, M Fatenejad, M Koenig, Y Kuramitsu, CC Kuranz, DQ Lamb, D Lee, MJ MacDonald, CD Murphy, H-S Park, A Pelka, A Ravasio, Y Sakawa, AA Schekochihin, A Scopatz, P Tzeferacos, WC Wan, NC Woolsey, R Yurchak, B Reville, G Gregori


Letter of Intent: The Precision IceCube Next Generation Upgrade (PINGU)

ArXiv (2014)

TI-P Collaboration

The Precision IceCube Next Generation Upgrade (PINGU) is a proposed low-energy in-fill extension to the IceCube Observatory. With detection technology modeled closely on the successful IceCube example, PINGU will feature the world's largest effective volume for neutrinos at an energy threshold of a few GeV, enabling it to reach its chief goal of determining the neutrino mass hierarchy (NMH) quickly and at modest cost. PINGU will be able to distinguish between the normal and inverted NMH at $3\sigma$ significance with an estimated 3.5 years of data. With its unprecedented statistical sample of low energy atmospheric neutrinos, PINGU will also have highly competitive sensitivity to $\nu_\mu$ disappearance, $\theta_{23}$ octant and maximal mixing, and $\nu_\tau$ appearance. PINGU can also extend the search for solar WIMP dark matter into the region currently favored by some direct dark matter experiments. At the lower end of the energy range, PINGU can use neutrino tomography to perform the first-ever direct measurement of the composition of the Earth's core. With its increased module density, PINGU will improve IceCube's sensitivity to galactic supernova neutrino bursts and enable it to extract the neutrino energy spectral shape.


Particle Acceleration by Shocks in Supernova Remnants

BRAZILIAN JOURNAL OF PHYSICS 44 (2014) 415-425

AR Bell


Galaxy merger histories and the role of merging in driving star formation at z>1

ArXiv (2014)

S Kaviraj, J Devriendt, Y Dubois, A Slyz, C Welker, C Pichon, S Peirani, DL Borgne

We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M > 10^10 MSun) galaxies around the epoch of peak cosmic star formation (1<z<4). The fraction of massive galaxies in major mergers (mass ratio R<4:1) is around 3%, a factor of ~2.5 lower than minor mergers (4:1<R <10:1) at these epochs, with no trend with redshift. At z~1, around a third of massive galaxies have undergone a major merger, while all such systems have undergone either a major or minor merger. While almost all major mergers at z>3 are 'blue' (i.e. have significant associated star formation), the proportion of 'red' mergers increases rapidly at z<2, with most merging systems at z~1.5 producing remnants that are red in rest-frame UV-optical colours. The star formation enhancement during major mergers is mild (~20-40%) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (R < 10:1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard LCDM paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today's stellar mass was formed.


A NEW STELLAR CHEMO-KINEMATIC RELATION REVEALS THE MERGER HISTORY OF THE MILKY WAY DISK

ASTROPHYSICAL JOURNAL LETTERS 781 (2014) ARTN L20

I Minchev, C Chiappini, M Martig, M Steinmetz, RS de Jong, C Boeche, C Scannapieco, T Zwitter, RFG Wyse, JJ Binney, J Bland-Hawthorn, O Bienayme, B Famaey, KC Freeman, BK Gibson, EK Grebel, G Gilmore, A Helmi, G Kordopatis, YS Lee, U Munari, JF Navarro, QA Parker, AC Quillen, WA Reid, A Siebert, A Siviero, G Seabroke, F Watson, M Williams


Resolving ultrafast heating of dense cryogenic hydrogen.

Physical review letters 112 (2014) 105002-

U Zastrau, P Sperling, M Harmand, A Becker, T Bornath, R Bredow, S Dziarzhytski, T Fennel, LB Fletcher, E Förster, S Göde, G Gregori, V Hilbert, D Hochhaus, B Holst, T Laarmann, HJ Lee, T Ma, JP Mithen, R Mitzner, CD Murphy, M Nakatsutsumi, P Neumayer, A Przystawik, S Roling, M Schulz, B Siemer, S Skruszewicz, J Tiggesbäumker, S Toleikis, T Tschentscher, T White, M Wöstmann, H Zacharias, T Döppner, SH Glenzer, R Redmer

We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300  fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9  ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory.


Erratum: Long-wavelength limit of gyrokinetics in a turbulent tokamak and its intrinsic ambipolarity (Plasma Phys. Control. Fusion (2012) 54 (115007))

Plasma Physics and Controlled Fusion 56 (2014)

I Calvo, FI Parra, FI Parra


Self-consistent flattened isochrone models

ArXiv (2014)

J Binney

We present a family of self-consistent axisymmetric stellar systems that have analytic distribution functions (DFs) of the form f(J), so they depend on three integrals of motion and have triaxial velocity ellipsoids. The models, which are generalisations of Henon's isochrone sphere, have four dimensionless parameters, two determining the part of the DF that is even in L_z, and two determining the odd part of the DF (which determines the azimuthal velocity distribution). Outside their cores, the velocity ellipsoids of all models tend to point to the model's centre, and we argue that this behaviour is generic, so near the symmetry axis of a flattened model, the long axis of the velocity ellipsoid is naturally aligned with the symmetry axis and not perpendicular to it as in many published dynamical models of well-studied galaxies. By varying one of the DF's parameters, the intensity of rotation can be increased from zero up to a maximum value set by the requirement that the DF be non-negative. Since angle-action coordinates are easily computed for these models, they are ideally suited for perturbative treatments and stability analysis. They can also be used to choose initial conditions for an N-body model that starts in perfect equilibrium and to model observations of early-type galaxies. The modelling technique introduced here is readily extended to different radial density profiles, more complex kinematics, and multi-component systems. A number of important technical issues surrounding the determination of the models' observable properties are explained in two appendices.


KINEMATIC MODELING OF THE MILKY WAY USING THE RAVE AND GCS STELLAR SURVEYS

ASTROPHYSICAL JOURNAL 793 (2014) ARTN 51

S Sharma, J Bland-Hawthorn, J Binney, KC Freeman, M Steinmetz, C Boeche, O Bienayme, BK Gibson, GF Gilmore, EK Grebel, A Helmi, G Kordopatis, U Munari, JF Navarro, QA Parker, WA Reid, GM Seabroke, A Siebert, F Watson, MEK Williams, RFG Wyse, T Zwitter


Erratum: "Correlations at large scales and the onset of turbulence in the fast solar wind" (2013, ApJ, 778, 177)

Astrophysical Journal 782 (2014)

RT Wicks, DA Roberts, A Mallet, AA Schekochihin, TS Horbury, CHK Chen